Following coronary artery occlusion, the changes in myocardial blood flow, heart rate and blood pressure were correlated with ST segment elevation in the electrocardiogram (an index of tissue injury). Regression equations were developed which showed that 1) ischemic injury is determined by the balance of blood flow (oxygen supply) and work load (oxygen demand), 2) changes in these functions during ischemia can modify ischemic injury, and 3) collateral blood flow is influenced positively by arterial pressure and negatively by heart rate.